The vertical cavity surface emitting laser (VCSEL) has become an important light source within many technological fields, such as optical telecommunications and sensing. VCSELs are attractive since they can have low threshold currents, low power consumption and high quality far field patterns. Further, VCSELs can have a low manufacturing and testing cost due to their small wafer footprint and the ability to perform quality assessment at wafer scale.
The optical cavity of a VCSEL is typically short, and so it is inherently only capable of lasing on a single longitudinal mode of the laser's optical cavity. However, the transverse dimensions of the optical cavity are typically considerably larger than the length of the optical cavity. Consequently the emitted output beam may comprise a plurality of transverse modes, including a fundamental mode and higher order modes.
Different transverse modes of a VCSEL have different lateral extensions. Consequently, different transverse modes typically experience different round-trip cavity losses. Further the current density within the active layer is typically lower within the centre of the VCSEL than away from the centre. This can be particularly pronounced in VCSELs having both a top electrode with an emission window and a current injection aperture within the laser cavity that is narrower than the emission window. Consequently the optical gain within the active layer will be inhomogeneous, and different transverse modes will experience different levels of optical gain.
The round trip cavity losses and gains for different transverse modes determine which transverse mode or modes preferentially lase. The losses and gains vary with the current injection and ambient conditions of the VCSEL, and consequently which transverse modes lase can vary. The problem can be exacerbated by two further factors as laser drive current increases. Firstly, due to spatial hole burning the exitation of existing lasing modes will typically not increase in amplitude as quickly as other modes. Secondly, as new modes commence lasing and grow in excitation they may cannibalise existing lasing modes.
Different transverse modes lase at slightly different wavelengths, and consequently in uncontrolled or variable multimode operation the optical bandwidth of the VCSEL output is broader than in single mode operation. However, many VCSEL applications require a narrow optical bandwidth output. In particular the transmission of high data rate telecommunications signals requires narrow optical bandwidths in order to minimize the mode dispersion that occurs during transmission along lengthy optical fibres, which degrades the shape of the optical data signals carried by the light.
Furthermore when the VCSEL is able to lase in different transverse modes the operation can be unstable, with different transverse modes dominating the output under different operating conditions. This instability can introduce intensity noise into transmitted signals carried by the VCSEL. Consequently, there is a need to control the lasing behaviour of VCSEL modes.
The fundamental transverse cavity mode of a typical VCSEL has a circular cross-section and an intensity profile with a single central peak. By contrast the higher order transverse modes typically comprise an even number of peaks arranged around a ring in a pattern having n-fold rotational symmetry. These higher order transverse modes have a broader lateral extension than the fundamental mode. Consequently an approach to stabilising the output of a VCSEL and reducing its bandwidth is to introduce lateral attenuation elements into the laser cavity, which preferentially suppress the higher order modes with respect to the fundamental mode, to cause single fundamental transverse mode operation of the laser. Examples of such approaches include the designs disclosed in EP1276188 and EP1496583.
The lateral width of the fundamental transverse mode is narrow, which limits the optical output power that a single fundamental mode VCSEL is capable of producing. Consequently to achieve higher optical output powers it would be desirable to drive one or more higher order transverse modes of the VCSEL, whilst at least partially limiting the range of driven modes, thereby narrowing the output optical bandwidth compared with an unlimited design and reducing instability.
EP1835577 discloses a design of VCSEL comprising a pit that is etched into the centre of the upper reflector of the laser cavity. The pit modifies the reflectivity of the upper reflector of the laser cavity so as to preferentially suppress modes that optically overlap the pit, in particular the fundamental mode, thereby narrowing the bandwidth of the optical output.
However, the production of the etch pit of EP1835577 requires several additional lithographic processing steps within the VCSEL manufacturing process, such as photoresist deposition, photolithographic exposure, photoresist development, etching and resist removal. Such extra steps increase the complexity of the manufacturing process, and inaccuracies reduce the reliability and attendant yield of that manufacturing process.
U.S. Pat. No. 6,990,128 discloses an alternative design in which a surface of the laser cavity is selectively etched with a pattern of holes or grooves. The patterns of holes or grooves modify the reflectivity of the upper reflector of the laser cavity. The patterned reflectivity of the upper reflector suppresses transverse modes that optically overlap with the etched pattern, thereby preferentially enhancing a lasing mode in which the intensity profile corresponds with the gaps between the etched regions. Such a design seeks to enable a VCSEL to be driven on substantially a single higher order transverse mode.
However, the production of the etch pattern of U.S. Pat. No. 6,990,128 requires several additional lithographic processing steps within the VCSEL manufacturing process. Such extra steps increase the complexity of the manufacturing process, and inaccuracies reduce the reliability and attendant yield of that manufacturing process.
In particular variations in the dimensions and alignment of the etch pattern will affect the performance of the manufactured VCSELs.
Thus, there remains in the industry a need for an easily manufacturable VCSEL that provides a narrow bandwidth output, and which seeks to mitigate at least some of the problems of prior art VCSELs.